1. Field of the Invention
This invention relates to catalytic filters comprising a silicon carbide (“SiC”) foam and an active phase comprising at least one metallic element, processes for preparing the catalytic filters and methods for purification of exhaust gases from internal combustion engines, particularly diesel engines, using the catalytic filters.
2. Description of Related Art
In an ideal case, the exhaust gases from a hydrocarbon internal combustion engine will only contain carbon dioxide (“CO2”) and water (“H2O”). In reality, the formation of other gases and solid products is observed. This is partly due to the presence of impurities contained in the hydrocarbons (such as sulphur compounds) and partly due to the complexity of the chemical reactions during combustion. For example, it is known that combustion in a diesel engine is non-homogenous and results in temperatures that are highly variable from one point of the fuel jet to another. The fuel jet may also be locally turbulent, which considerably complicates the analysis and forecasting of chemical reactions during the engine design. Therefore, during the combustion of hydrocarbons such as gasoline or gasoil in an internal combustion engine, there are firstly gas releases such as carbon monoxide (“CO”), nitrogen oxides (represented by the formula “NOx” and containing mainly molecules such as nitrogen oxide (“NO”) and nitrogen dioxide (“NO2”)), unburned hydrocarbons, CO2, H2O, together with the emission of variable sized solid particles.
Three different problems have been encountered with the attempted purification of exhaust gases from diesel engines. The first problem relates to the conversion of CO, a toxic and explosive gas, into CO2. The second problem relates to the conversion of NOx derived from the reaction between nitrogen and oxygen contained in air and that have an irritating effect on the mucous membranes of the respiratory system, into nitrogen. The third problem relates to the formation of solid particles during combustion. In particular, solid particles formed during combustion may contain soot and condensed heavy hydrocarbons and mineral compounds such as sulphates present in the fuel. Their size varies as a function of the engine speed and the temperature in the combustion chamber; the smallest particles can penetrate deep into the lungs, bronchial tracts and alveoli, thus reducing lung capacity. Mutagenic and carcinogenic effects, particularly due to soot and condensed heavy hydrocarbon particles (such as aromatic polynuclide molecules) are known. Exhaust gases may also contain unburned fuel. Very small solid particles may also form in other types of internal combustion engines, and particularly other types of engines using liquid fuels. Thus, the destruction of emitted solid particles is a critical problem, particularly for diesel engines. It has been observed that the formation of solid particles during combustion is more specific to, but not limited to, diesel engines that use a heavier fuel and have an operating temperature lower than that of gasoline engines.
Several different approaches can be used to solve these problems. For example, a combustion speed can be found that will minimize the formation of undesirable waste releases. Further, this reduction at the source may be substituted or complemented by catalytic converters. However, catalytic converters used to minimize NOx in gasoline engines do not necessarily give good results for diesel engines since the oxygen content and the exhaust gas temperature of a diesel engine and a gasoline engine are different.
Additionally, filtration systems designed to retain particles in diesel engine exhaust gases can be used. For example, a system of filters using metallic or ceramic wires, for example in the form of a sponge coated with a catalytic deposit, is known. However, this system is fairly sensitive to vibrations that tend to wear the catalytic deposit; the catalytic deposit forms dust that is emitted with the exhaust.
Another known filtration system uses ceramic foam filters, typically based on extruded silicon carbide (“SiC”) or extruded cordierite, comprising a fairly narrow distribution of large pores (diameter of the order of 100 μm to 500 μm) with few closed channels. These filters are characterized by good retention capacity before clogging and better crack resistance, but they suffer from a high pressure loss. They may be fabricated in the form of filter cartridges or filter inserts, but they are expensive.
Monolithic foam structure filters are also known for which a large porosity is used at the filter inlet with a smaller porosity at the outlet, the porosity possibly being varied continuously or discretely, on a single monolith with different porosity areas or by putting successive filters adjacent to each other each with a different porosity. For example, EP 0 050 340 (Bridgestone Tire Co. Ltd.), FR 2 498 471 (W.R. Grace & Co.), U.S. Pat. No. 4,912,076 (Swiss Aluminum Ltd.), and FR 2 650 628 (Aris s.a.) describe how to prepare particle filters from different filter elements made of ceramic material, each with a different porosity, such that the gases to be purified firstly pass through an area with large pores, and then through an area with smaller porosity. Additionally, U.S. Pat. No. 4,857,088 (Swiss Aluminum Ltd.) describes a particle filter with a more complex non-uniform type of porosity, but which is still based on the same principle of using a filter formed from filter elements made of ceramic material each with a different porosity. U.S. Pat. No. 5,053,062 (Donaldson Co.) proposes the use of a filter disk with large pores at the input to a filter cartridge, the filter disk having a high thermal emissivity such that the combustion of large carbon particles collected on this disk contributes to the increase in temperature of the filter cartridge.
One particular type of known filters with non-uniform porosities is honeycomb type filters in which the porous wall is covered on the outlet surface by a thin membrane with a finer porosity designed to facilitate filtration of very fine particles without significantly increasing the pressure loss. For example, U.S. Pat. No. 4,846,906 (The Durion Company) describes filters for diesel engine exhaust gases including a filter body based on ceramics with an open porosity, covered by a ceramic membrane with an open porosity with an average pore size smaller than the size of the filter body. Ceramics are prepared from an aluminosilicates gel. U.S. Pat. No. 4,871,495 (The Durion Company) describes a thermal process for preparing ceramics, particularly cordierite based ceramics, with a controlled pore size.
Additional honeycomb type filters are described in Patent Application WO 00/01463 (Silentor Notox) which teaches a filter for diesel engine exhaust gases composed of a filter element with a honeycomb structure, with a pore size of the order of 35-500 μm, and filter elements with a smaller pore size, 5-10 μm in an intermediate zone, and 0.5-5 μm in the outer zone that the gases pass through last. The application teaches that this type of filter element may be made of ceramic material based on SiC, prepared by extrusion of SiC powders, part of which must have a very fine grain size (of the order of 0.1-10 μm) so that it can act as a binder. The application further recommends use of an alumina wash-coat type coating.
A description of “diesel filter” state of the art is provided by P. Degobert in the article “Pollution atmosphérique. Post-traitements (Atmospheric pollution. Post-treatments)”, published in May 1995 in the treatise “Mécanique et Chaleur (Mechanics and Heat)”, volume BL1, booklet B 2 711 in the “Techniques de l'Ingénieur (Engineering techniques)” collection.
All the known systems involve the problem of regeneration. Particles captured in the filter block the pores, which increases the pressure loss. Therefore, the particles have to be removed by burning them either continuously or discontinuously. The temperature of exhaust gases from a diesel engine is too low for direct combustion of captured soot or hydrocarbon particles; their temperature is usually lower than 400° C., while spontaneous combustion of the particles takes place at a minimum temperature of about 400° C. to about 800° C. (depending on the particle composition). Therefore, a catalyst and/or heat have to be added to achieve permanent or periodic combustion of the captured particles. The addition of heat requires sophisticated temperature control, since materials used in currently available systems are poor conductors of heat. There are several regeneration systems, in series (with a single filter) or in parallel (with at least two filters) with or without added air, with addition of heat by electrical heating or by torch. However, these filter systems are complex and expensive, and require complex regulation systems.
For example, a filter system used on tourism vehicles uses very finely ground SiC based cartridges extruded as a honeycomb and sintered at a temperature of more than 1500° C. In these filters, typically one channel out of two is blocked which encourages the passage of gases through the ceramic pores rather than through the channel's system. Particles retained by the filter are periodically burned by adding a catalyst to the fuel. This system is efficient but expensive. Patent application EP 1 225 311 A2 (Th. J. Heimbach GmbH) describes a filter device made of an α-SiC “honeycomb” type ceramic.
The document by P. Degobert mentioned above describes another system consisting of a ceramic foam filter made from silicon carbide or cordierite with a fairly narrow distribution of large quasi-circular pores with a diameter of 250 to 500 μm, with a winding in-depth path, with few closed channels. These filters are made by impregnation of a polyurethane foam matrix by a cordierite paste that is then calcined. The result is thus a cordierite foam with about 20-30 pores per cm3. These filters, made in the form of cartridges or filter inserts, have a retention ratio of the order of 60 to 70%.
Another known filter system is based on an extruded honeycombed cordierite cartridge impregnated with precious metals such as platinum.
European Patent 0 160 482 B1 (Engelhard Corporation) describes a filter composed of a cartridge made of ceramic material with porous walls impregnated with a catalyst composed of a mix of an element in the platinum group and an oxide of an element belonging to the alkaline earths. In this filter, the catalyst reduces the combustion temperature of soot particles, which are therefore continuously eliminated. The cartridge may be composed of cellular or monolithic ceramic material.
Patent application EP 1 142 619 A1 (Ibiden) describes a diesel filter system, in which the ceramic filter medium consists of a sintered porous SiC with an average pore diameter of about 5 to 15 μm, in which at least 20% of the pores are open. This filter medium is made from a mix of α-SiC and β-SiC powders prepared with an organic binder, or from silicon nitride, sialon, alumina, cordierite or mullite. Several of these ceramic blocks are assembled using a ceramic fiber paste based on aluminium silicate. This avoids the need to use relatively large blocks, since the probability of cracks forming in the ceramic material increases with the block size.
French patent application 2 818 163 (Renault) describes a new copper based catalyst in which soot particles can be burned at normal exhaust gas temperatures, namely about 300° C. This catalyst may be applied on known ceramic supports, particularly oxide type supports such as cordierite, or on metallic filters.
Patent application WO 93/13303 (Stobbe) describes a filter system made of α-SiC sintered at 2200-2600° C. composed of segments that can be heated individually or in groups, by the Joule effect, in order to burn soot particles. The electrical resistance of this product is fairly high, and a high current is necessary to heat it.
Patent application JP 07-080226 (Ibiden) proposes to reduce the electrical resistance of SiC ceramics by adding additives.
The article “An optimal NOx assisted abatement of diesel soot in an advanced catalytic filter design” by A. Setiabudi, M. Makkee and J. A. Moulijn, published in the Applied Catalysis B review: Environmental, vol 42, P. 35-45 (2003) describes a catalyst prepared by impregnation of a 20 ppi SiC foam with a solution of Pt (NH3)4 Cl2H2O, leading to a Pt content of 1.5%.
Different methods of making SiC are known. For example, patent EP 313 480 B1 (Pechiney Electrométallurgie) describes a process for production of fine silicon carbide grains consisting of generating SiO vapors in a first reaction area by heating a mix of SiO2+Si to a temperature ranging from 1100 to 1400° C. at a pressure ranging from 0.1 to 1.5 hPa, and then bringing these SiO vapors into contact with reactive carbon with a specific surface area equal to at least 200 m2/g at a temperature ranging from 1100 to 1400° C. A variant of this process is described in patent EP 543 752 B1 (Pechiney Recherche). The process described in this patent consists of preparing a carbon foam by pyrolysis of a polyurethane foam impregnated with a thermosetting resin at a temperature ranging from 700 to 900° C., activating the foam by a CO2 draft at 700-1000° C., and then exposing this foam to an SiO vapour to form an SiC foam.
Patent EP 440 569 B1 (Pechiney Recherche) describes a process for obtaining SiC consisting of mixing furfurylic resin with an organic hardener and silicon powder, hardening this mix in a drying oven at about 100 to 120° C., carbonizing this hardened mix by heating it to a temperature of the order of 900° C. under a nitrogen draft, and then carbiding this intermediate prouct by heating it to a temperature of the order of 1200° C. under an argon draft, possibly followed by elimination of excess carbon at a temperature of about 600° C. under air. Patent EP 511 919 B1 (Pechiney Recherche) describes the preparation of catalysts starting from this product.
Patent EP 624 560 B1 (Pechiney Recherche) describes a process for obtaining SiC consisting of impregnating a polyurethane foam with a suspension of silicon powder in an organic resin with a controlled mass ratio, polymerising the resin, carbonising the organic polymers and then carbiding the silicon. According to the information in this patent, it is preferred to use a specific surface area BET less than 5 m2/g and a very low mesoporosity for the filtration of diesel engine exhaust gases.
SiC based foams with different pore sizes, for example macropores and micropores, are also known. This type of foam, prepared using different processes, is disclosed in patents FR 2 766 389 (Pechiney Recherche) that describes a foam with a porosity with dual mode distribution, and in patent FR 2 705 340 (Pechiney Recherche).
In any case, in the current state of the art, the use of a Diesel filter introduces a non-negligible extra cost and consequently only a few vehicle models are equipped with a Diesel filter.
In view of the problems related to clogging, resistance to vibrations and temperature cycles, regeneration, regulation and cost of filters according to the state of the art, active research is being done to find simpler, more robust and less expensive filter devices. This type of device would have to use a filter medium that is easy to make in different geometric shapes, and that does not crack during manufacturing or manipulation.